* He whistled a happy tune as he walked down the street. (This shows someone making a sound with their mouth)
* The wind whistled through the trees, creating a haunting melody. (This uses "whistle" to describe the sound the wind makes)
* She whistled sharply, signaling the others to be quiet. (This uses "whistle" to describe a specific type of whistle sound)
* The referee whistled for a penalty. (This shows how a whistle is used in a specific context)
* The boy whistled at the pretty girl, but she just ignored him. (This uses "whistle" in a more informal, almost slang way)
As you can see, "whistle" can be used in a variety of ways to describe both the act of whistling and the sound itself.